1. Field of the Invention
The present invention relates to a molded article of foamed and expanded beads of a propylene resin, which has excellent mechanical strength and a low expansion ratio.
2. Description of the Background Art
Molded articles of foamed and expanded beads (hereinafter referred to as "foamed beads" merely) of a propylene resin, which are obtained by heating and molding the foamed beads of the propylene resin in a mold, are widely used in various fields. In recent years in particular, molded articles of foamed beads (hereinafter referred to as the "expansion-molded articles"), which have a relatively low expansion ratio of about 2 to 8 times, have been expected to be in demand in fields required to be thin-wall and have high stiffness and energy absorption performance. There is a possibility that they may be widely used in fields of application of, for example, cores for integrally molded articles with a skin, such as automotive door panels, pillars and instrument panels, jack and tool housings, etc.
There have heretofore been known foamed beads of propylene resins having a relatively low expansion ratio and molded articles obtained by using such foamed beads. For example, Example 1 of Japanese Patent Publication No. 43493/1984 describes the fact that foamed beads of a polypropylene resin having an apparent expansion ratio of 5 times (apparent density: about 0.18 g/cm.sup.3) were molded in a mold to obtain a molded article which had good fusion-bonded state and appearance and was free of shrinkage. Example 2 of Japanese Patent Publication No. 33253/1987 describes the fact that foamed beads of a polypropylene resin having an expansion ratio of 5 times were charged into a mold and molded therein to obtain a molded article having high mechanical strength. Japanese Patent Application Laid-Open No. 4738/1986 describes a process for producing foamed beads of a polypropylene resin, which are used for expansion molding, and Run Nos. 1, 5 and 6 in Table 3 of this publication show examples where foamed beads of the polypropylene resin having bulk expansion ratios of 5 times (apparent density: about 0.270 g/cm.sup.3), 3 times (apparent density: about 0.450 g/cm.sup.3) and 7 times (apparent density: about 0.193 g/cM.sup.3), respectively, were produced. Incidentally, the bulk expansion ratio is a value determined by placing a plurality of the foamed beads in a graduated cylinder, dividing the weight of the foamed beads by a volume read by the graduated cylinder at this time to find a bulk density (g/cm.sup.3) of the foamed beads, and dividing the density (g/cm.sup.3) of the base resin of the foamed beads by the thus-found bulk density of the foamed beads.
Further, Example 5 of Japanese Patent Application Laid-Open No. 372630/1992 describes the fact that a propylene resin was used to obtain foamed beads having an expansion ratio of 2.6 cm.sup.3 /g (apparent density: about 0.385 g/cm.sup.3). Example 1 of Japanese Patent Application Laid-Open No. 176077/1998 describes the fact that foamed beads of a propylene resin having an average expansion ration as low as 3.6 times (apparent density: about 0.250 g/cm.sup.3) were obtained.
However, Japanese Patent Publication No. 43493/1984, Japanese Patent Publication No. 33253/1987 and Japanese Patent Application Laid-Open No. 4738/1986 do not describe anything about the fact that a molded article, wherein a value (hereinafter may be referred to as the "ratio of the bending strength to the density" merely) obtained by dividing its maximum bending strength (kgf/cm.sup.2) determined in accordance with JIS K 7221 by its apparent density (g/cm.sup.3) is at least 155 (kgf/cm.sup.2)/(g/cm.sup.3), is obtained. Moreover, it has been hard to say that when such foamed beads having a low expansion ratio as described in these publications are molded to use the resultant molded articles as such cores for integrally molded articles with a skin, and jack and tool housings as described above, their strength is sufficient. The foamed beads having an expansion ratio of 2.6 cm.sup.3 /g described in Example 5 of Japanese Patent Application Laid-Open No. 372630/1992 are further expanded to provide foamed beads having an expansion ration of 13.56 cm.sup.3 /g (apparent density: about 0.074 g/cm.sup.3). Therefore, the foamed beads are not those used for obtaining expansion-molded articles having a low expansion ratio. The foamed beads having a relatively low average expansion ratio of 1.5 to 4.7 times described in Japanese Patent Application Laid-Open No. 176077/1998 are also further expanded to provide foamed beads having an apparent density lower than 0.11 g/cm.sup.3, and the foamed beads thus provided are then molded. Therefore, the expansion-molded articles described in Japanese Patent Application Laid-Open No. 176077/1998 are also different from the expansion-molded article of the propylene resin having a low expansion ratio that the present invention has as its object the provision thereof.
When resin particles are expanded, the expansion ratio of the resulting foamed beads becomes lower as the pressure of a foaming agent fed into a vessel in which the resin particles have been dispersed into dispersion medium is lower (the amount of the foaming agent is less). However, foamed beads of a propylene resin having low expansion ratio obtained in such a manner involve problem that the expandability and fusion bonding property upon the molding of the foamed beads become more deteriorated as the expansion ratio is lower. As a result, the appearance of the resulting molded article is deteriorated, and moreover it is impossible to obtain an excellent expansion-molded article in which good physical properties such as stiffness, that a foam having a low expansion ratio has, have been sufficiently introduced.